The hydrogen-absorbing alloy is noticed as an energy-converting material and an energy storing material because it has a property capable of safely and easily absorbing hydrogen. Further, the hydrogen-absorbing alloy is utilized in the following various fields as one of new functional materials.    (1) storage-transport of hydrogen, storage-transport of heat    (2) conversion of heat-mechanical energy    (3) separation-purification of hydrogen    (4) separation of hydrogen isotope    (5) cell containing hydrogen as an active substance    (6) catalyst in synthetic chemistry    (7) Temperature sensor.
Thus, the hydrogen-absorbing alloy has a possibility for various mechanical, physical and chemical applications, so that it is a very important material as one of key materials in the future and novel industries.
Particularly, a nickel-hydrogen secondary cell using the hydrogen-absorbing alloy as a negative electrode material is noticed as a next-generation commercial cell. Because, the nickel-hydrogen secondary cell has merits that (a) it is high in the capacity, (b) it is strong to the over-charge and over-discharge, (c) high charge-discharge are possible, (d) it is clean, (e) it is inter-exchangeable with a nickel-cadmium cell, and the like.
The hydrogen-absorbing alloy means a metal or an alloy capable of forming a stable compound with hydrogen, which includes a single type of Pd, Ti, Zr, V, a rare earth metal element (hereinafter abbreviated as REM simply), an alkaline earth element or the like, and an alloy type of forming an alloy with such a metal element and the other metal element.
Among them, the alloy type has features that (i) the bonding force between metal and hydrogen is properly weakened to relatively easily conduct not only the absorption reaction of hydrogen but also the dissociation reaction; (ii) the magnitude of equilibrium hydrogen pressure required for absorption-discharge reaction (plateau pressure), breadth of equilibrium region (plateau region), change of equilibrium pressure in the course of absorbing hydrogen (flatness) and the like are easily improved; (iii) the chemical and physical stabilities are high; and the like. Moreover, as the alloy type hydrogen-absorbing alloy are known a rare earth system (LaNi5, MmNi5 or the like), a Laves system (ZrV2, ZrMn2 or the like), a titanium system (TiNi, TiFe or the like), a Mg system (Mg2Ni, MgNi2 or the like) and Mg-REM-Ni alloy.
As regards the above alloy type hydrogen-absorbing alloy, the rare earth based hydrogen-absorbing alloy such as LaNi5, MmNi5 or the like is used as an electrode material for a cell. However, the conventional rare earth based hydrogen-absorbing alloy has a discharge capacity previously exceeding 80% of theoretical capacity, so that there is a limit in the increase of the capacity at the present time. Under such a situation, JP-A-H11-323469 discloses that the Mg-REM-Ni based alloys are noticed as a hydrogen-absorbing alloy effective for further increasing the capacity and it is expected to increase the demand thereof in future.